The present invention relates to semicrystalline propylene polymer compositions which are particularly suitable for producing biaxially oriented films. The invention further relates to the use of the semicrystalline propylene polymer compositions for producing films, fibers or moldings, and also to the films, fibers and moldings made from these compositions.
The term polypropylene is generally understood to denote a wide variety of different polymers, a common feature of which is that they have been built up to a substantial extent from the monomer propylene. The various polypropylenes are generally obtained by coordinative polymerization on catalysts made from transition metals, which give predominantly ordered incorporation of the monomers into a growing polymer chain.
The polymer chains obtained during the polymerization of propylene with the usual coordination catalysts have a methyl side group on each second carbon atom. The polymerization therefore proceeds in a regioselective manner. Depending on the orientation of the monomers during incorporation into the chain, various stereochemical configurations are obtained. If the monomers all have the same arrangement when they are incorporated, the methyl side groups in the polymer chain are then all on the same side of the principal chain. The term used is isotactic polypropylene. If all of the monomers alternate in their spatial orientation when incorporated into the chain, the resultant polypropylene is termed syndiotactic. Both of these varieties with their stereoregular structures are semicrystalline and therefore have a melting point.
However, since the incorporation of the propylene monomers when coordination catalysts are used is not absolutely consistent, but some of the monomers are introduced in a way which differs from that of the majority, the polymer chains formed always have xe2x80x9cdefectsxe2x80x9d in the prevailing arrangement, and the number of these defects can vary considerably.
The longer the defect-free structure sequences in the polymer chains, the more readily the chains crystallize and therefore the higher are the crystallinity and the melting point of the polypropylene.
If the methyl side groups have an irregular stereochemical arrangement the polypropylenes are termed atactic. These are completely amorphous and therefore have no melting point.
The industrial preparation of polypropylene nowadays mostly uses heterogeneous catalysts based on titanium, and the resultant product is a predominantly isotactic polymer. These catalysts, for which the term Ziegler-Natta catalysts has become established, have a number of different centers active for polymerization. These centers differ both in their stereospecificity, i.e. in the number of xe2x80x9cdefectsxe2x80x9d which the resultant chains have, and also in the average molar mass of the chains formed. The predominant defects observed in all cases are stereo-defects, meaning that individual propylene monomers were incorporated syndiospecifically instead of isospecifically. The result of polymerization with heterogeneous catalysts of this type is therefore a mixture of various polymer chains which differ both in their stereochemistry and in their molar mass.
A substantial application sector for polypropylenes is that of films, particularly biaxially stretched films, frequently also termed BOPP (biaxially oriented polypropylene) films.
A general aim of almost all developments in the polypropylenes sector is to reduce the soluble fractions of the polymers used. This is frequently possible via the use of optimized conventional Ziegler-Natta catalysts. The result is firstly an improvement in organoleptic properties, advantageous for applications in the medical and food sectors, and secondly a favorable effect on mechanical properties, in particular stiffness. However, polypropylenes of this type with reduced soluble fractions cannot be used for producing biaxially stretched polypropylene films, since they have low capability, or no capability, for processing to give these films. Many efforts have therefore been made to use variations in the composition in order to find polypropylenes suitable for producing biaxially stretched polypropylene films.
EP-A 339 804 describes a mixture of a homopolypropylene and a random propylene copolymer, where the comonomer has been incorporated within the upper range of the molecular-weight distribution of the mixture. Mixtures of this type have good optical and mechanical properties, but have limited processibility.
EP-A 115 940 discloses propylene-ethylene copolymers suitable for producing biaxially stretched films and having from 0.1 to 2.0 mol % of ethylene and high isotacticity. These polymers have good extensibility, stiffness, transparency, impact strength and stability in relation to heat-shrinkage. However, they frequently do not meet the requirements of BOPP film producers with respect to mechanical, rheological and optical properties.
EP-A 657 476 describes an xcex1-olefin polymer obtained by polymerizing an a-olefin having 3 or more carbon atoms and whose composition has been defined via the proportions by weight of fractions soluble in xylene at 20xc2x0 C. and insoluble in xylene at 105xc2x0 C.
JP-A 10 053 675 describes a polypropylene composition composed of a high-molecular-weight crystalline polypropylene with a soluble fraction of less than 5% and a low-molecular-weight polyolefin composition with a soluble fraction of more than 30%.
Although the propylene polymer compositions known from the prior art permit the production of biaxially oriented polypropylene films, they do not combine this property with ideal processibility and very good mechanical properties of the films. This means that it has hitherto not been possible to decouple the inverse correlation between processibility and mechanical properties of the films.
It is an object of the present invention, therefore, to develop propylene polymer compositions which have excellent processibility to give biaxially stretched films and from which, at the same time, films with very good mechanical and optical properties can be produced. It should also be possible to obtain these by a very uncomplicated process, and the films should have good barrier action, for example with respect to oxygen and water vapor.
We have found that this object is achieved by a semicrystalline propylene polymer composition with good suitability for producing biaxially oriented films and prepared by polymerizing propylene, ethylene and/or C4-C18-1-alkenes, where at least 50 mol % of the monomer units present arise from the polymerization of propylene,
and with a melting point TM of from 65 to 170xc2x0 C., where the melting point TM is determined by differential scanning calorimetry (DSC) to ISO 3146 by heating a previously melted specimen at a heating rate of 20xc2x0 C./min, and is measured in xc2x0 C. and is the maximum of the resultant curve,
and where the semicrystalline propylene polymer composition can be broken down into
from 40 to 85% by weight of a principal component A,
from 0 to 55% by weight of an ancillary component B, and
from 0 to 55% by weight of an ancillary component C,
where the proportions of components A, B and C are determined by carrying out TREF (temperature rising elution fractionation) in which the polymers are firstly dissolved in boiling xylene and the solution is then cooled at a cooling rate of 10xc2x0 C./h to 25xc2x0 C., and then, as the temperature rises, that fraction of the propylene polymer composition which is soluble in xylene at (TM/2)+7.5xc2x0 C. is then dissolved and separated off from the remaining solid, and then, as the temperature rises, at all of the higher temperatures 70xc2x0 C., 75xc2x0 C., 80xc2x0 C., 85xc2x0 C., 90xc2x0 C., 94xc2x0 C., 98xc2x0 C., 102xc2x0 C., 107xc2x0 C., 112xc2x0 C., 117xc2x0 C., 122xc2x0 C. and 125xc2x0 C. the fractions soluble within the temperature range between this elution temperature and the preceding elution temperature are eluted, and the fractions taken into consideration during the evaluation which follows are those whose proportion by weight is at least 1% by weight of the initial weight of the propylene polymer composition specimen, and gel permeation chromatography (GPC) at 145xc2x0 C. in 1,2,4-trichlorobenzene is used to measure the molar mass distribution of all of the fractions to be taken into consideration,
and the principal component A is formed by all of the fractions which are eluted at above (TM/2)+7.5xc2x0 C. and have an average molar mass Mn (number average)xe2x89xa7120,000 g/mol,
the ancillary component B is formed by the fraction which is eluted at (TM/2)+7.5xc2x0 C., and
the ancillary component C is formed by all of the fractions to be taken into consideration and which are eluted at above (TM/2)+7.5xc2x0 C. and have an average molar mass Mn (number average) less than 120,000 g/mol,
and where at least one of the fractions forming the principal component A has a ratio between weight-average (Mw) and number-average (Mn) molar masses of the polymers Mw/Mn greater than 4.5.
In addition, semicrystalline propylene polymer compositions have been found which have good suitability for producing biaxially oriented films and are prepared by polymerizing propylene, ethylene and/or C4-C18-1-alkenes, where at least 50 mol % of the monomer units present arise from polymerizing propylene, and the compositions have a melting point TM of from 65 to 170xc2x0 C.,
where the semicrystalline propylene polymer composition can be broken down into
from 40 to 85% by weight of a principal component A,
from 15 to 55% by weight of an ancillary component B, and
from 0 to 40% by weight of an ancillary component C,
and the room-temperature xylene-soluble fraction XL of the semicrystalline propylene polymer composition is not more than 5% by weight.
The use of the semicrystalline propylene polymer composition for producing films, fibers or moldings has also been found, as have the films, fibers and moldings made from this composition.
The novel semicrystalline propylene polymer compositions are prepared by polymerizing propylene, ethylene and/or C4-C18-1-alkenes. For the purposes of the present invention, C4-C18-1-alkenes are linear or branched 1-alkenes which have from 4 to 18 carbon atoms. Preference is given to linear 1-alkenes, and particular mention is made of ethylene, 1-butene, 1-pentene, 1-hexene, 1-heptene and 1-octene and mixtures made from these comonomers, and preference is given to the use of ethylene or 1-butene. The propylene polymer compositions comprise at least 50 mol % of monomer units which arise from polymerizing propylene. The content of propylene-derived monomer units is preferably at least 70 mol % and in particular at least 85 mol %. In another preferred embodiment, propylene is the sole monomer used in preparing the novel propylene polymer compositions, meaning that the polymer is a propylene homopolymer. If use was made of one or more comonomers it may be that the entire propylene polymer composition has substantially the same comonomer distribution, like that of random copolymers. However, it may also be that, as in what are known as propylene impact copolymers, there is a mixture of different components which have different comonomer contents.
The novel semicrystalline propylene polymer compositions have melting points TM of from 65 to 170xc2x0 C., preferably from 135 to 165xc2x0 C.
For the purposes of the present invention, the melting point TM is the temperature of the maximum of the plot of enthalpy against temperature for a previously melted specimen heated at a heating rate of 20xc2x0 C./min obtained using differential scanning calorimetry (DSC) to ISO 3146. The DSC measurement here is usually carried out by first heating the specimen at a heating rate of 20xc2x0 C./min to about 40xc2x0 C. above the melting point, then allowing the specimen to undergo dynamic crystallization at a cooling rate of 20xc2x0 C./min and then determining the melting point TM during a second heating procedure at a heating rate of 20xc2x0 C./min.
To determine the proportions of components A, B and C in the semicrystalline propylene polymer compositions, according to the invention a fractionation is carried out using TREF (temperature rising elution fractionation) and the molar mass distribution of all of the fractions is then measured by gel permeation chromatography (GPC).
GPC and TREF are methods for using various physical properties to fractionate polymer specimens. While GPC fractionates polymer chains by their size, the separation in TREF is by crystallizability of the polymer molecules. The principle of temperature rising elution fractionation was described in detail in L. Wild, Advances in Polymer Sciences 98, 1-47 (1990), by way of example. In this technique, a polymer specimen is dissolved in a solvent at an elevated temperature, and the concentration of the solution should be below 2% by weight. The polymer solution is then cooled very slowly (about 0.1xc2x0 C./min). The first polymer molecules to precipitate are then those which crystallize very well, and these are followed by molecules with poorer crystallization properties. In the polymer particles produced in the solvent, therefore, the crystallizability of the molecules of which these particles are composed decreases from the inside toward the outside. The cooling is followed by the actual fractionation by heating the polymer suspension. During this process, the molecules which crystallize poorly, located on the periphery of the polymer particles, are first dissolved at a relatively low temperature and are removed with the solvent which has dissolved them, followed at a higher temperature by the polymer chains which crystallize more readily.